Eggs are a possibility, but we can comfortably exclude them from the equation because if eggs were the cause then the oil found on the shirt would be butter or margarine, neither of which makes such a stain.
Cereal and toast are ruled out because they don’t fit the requirements of the grease. This narrows the possibilities down to the range of breakfast foods that contain grease or other oils that would be found in the apartment of the typical college student. An oil of some kind, perhaps grease you know that he didn’t go out to eat at a diner or fast food joint, or any other restaurant for that matter because of the state of his breath, since he had access to a toothbrush it is safe to say that he partook of his breakfast at home. You look closely at the stain itself, at first glance it could be mistaken for water, but under scrutiny it is obvious that the stain was made by an oil of some kind, based on the nature of the spreading and absorption of the liquid into the fabric. Your observations lead you now to the stain itself, since there are no like stains on the shirt, particularly the shoulders it is safe to say once again that the substance was a result of ingestion of something. His face however his clean and on his breath is the powerful smell of mint tooth-paste. You want to determine what this stain is the collar on his shirt is ridged, and conforms only to one shape, the shirt being fairly new, thus you can deduce the obvious that since his head covers that part of the collar vertically meaning that nothing could have fallen from above him onto that spot unless he looked directly upwards, the stain most likely came as a result of something being orally ingested, and liquid accidentally landed upon the tip of his collar. Peradventure you notice while on an escapade to the facility of higher learning that your friend has a stain on the tip of the collar on his polo shirt. Practicality is everything in the art of deduction, thus I shall endeavor to make this of the utmost practicality to you by once again conjuring up a real world example in order to solidify the concepts in a more existential manner.
In essence rather than trying to determine what something is if you determine what it is not, obviously that narrows the possibilities of what it can be. First let me explain why the process of elimination is the utmost authority in making quick and accurate deductions the reason being that it can be supremely difficult to determine what something is, while at the same time it can be more expedient to do the opposite. It has come to my attention that the chiefest and most paramount part in the process of making deductions is the process of elimination the requirement of which is information, in copious amounts.